Optical preamplification is used in particular in high data rate receivers to amplify the signal in the optical domain rather than amplifying it in the electrical domain.
The main advantage of this is that the amplifier is a very low noise amplifier because optical amplification is based on movements of photons which givers rise to quantum noise, whereas electrical amplification is based on movements of electrons which gives rise to thermal noise, which is greater.
A diagram of a state-of-the-art optical preamplifier device for a receiver is shown in FIG. 1.
It is provided with an input E for the incident optical signal SE and with an output S delivering the amplified and detected signal. The input E is connected to a first optical amplifier 1 which is either a semiconductor optical amplifier (SOA) or an erbium-doped fiber amplifier (EDFA).
The amplifier is followed by a band-pass filter 2. The filter is tuned by means of a tuning loop B to the wavelength .lambda..sub.i of the incident signal SE.
The filter 2 is a tunable filter and it is tuned by the loop as a function of the wavelength .lambda..sub.i to pass the amplified signal SA without the broadband noise generated by the amplifier 1 (EDFA).
The amplified and filtered signal SF is detected by a photodetector such as a photodiode 3, and is optionally then electrically amplified 4 in the receiver.
The problem encountered by such an amplification device lies in the fact that the band-pass filter must be tuned to the wavelength .lambda..sub.i of the signal.
In practice, it is thus necessary to provide a tunable filter. The use of such filters raises numerous difficulties: they are complex to make, and they are costly; and also there are numerous problems that can give rise to poor tuning stability.
For such filters, complex filters are used that have negative feedback loops that enable the filters to remain locked on the wavelength of the incident signal, but such filters are costly and difficult to make in technical terms.